Method of withdrawing liquid from a pair of vertically spaced annular screens

ABSTRACT

A method and apparatus for selectively withdrawing liquor from digester cooking screens for controlling the temperature in continuous digesters. First and second pairs of screens, with cooperating headers, are provided, and first and second outlets spaced circumferentially around the digester are associated with each header. Each header provides a circumferential cavity free of dividing members. Within each pair of screens withdrawal is practiced so that each header is rested during each ultimate withdrawal step, and the side from which the liquor is drawn each successive withdrawal step that the same header is used is alternated. In practicing the invention, header blockage caused by deposited debris and scale is minimized and increased uniformity and improved product quality result.

This is a continuation of application Ser. No. 172,973 filed July 28,1980, now abandoned, which in turn is a continuation of Ser. No. 011,662filed Feb. 12, 1979, filed now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

Essentially all continuous digesters for producing pulp from cellulosicchips material contain screens for withdrawing liquor from the cookingportions of the digester, heating the withdrawn liquor to control thetemperature thereof, and returning the liquor to the cooking portion ofthe digester. Numerous different types of proposals have been made forthe exact manner of effecting the liquid withdrawal, such as shown inU.S. Pat. Nos. 2,695,232; 3,711,367; and 4,061,193. Many conventionalcommercial digesters have two upper cooking circulation screens and twolower cooking circulation screens, with associated headers, with oneoutlet extending from each header.

While conventional techniques for effecting cooking liquor recirculationare normally satisfactory, in larger digesters pulp uniformity andquality sometimes suffer as a result of problems in digesting liquorcirculation. This is particularly true for digesters having a diameterof about 14' or more.

According to the present invention, the sources of the problems withpulp quality and uniformity as a result of cooking liquor recirculationhave been identified, and pulp uniformity and quality have been improvedin a simple, economical and common effective manner. It has beenrecognized that as the distance from the central distribution pipe forthe cooking liquor recirculation to the digester wall increases, it iseasier for the liquor to find its own path as it moves through the chipsnear the wall area. It has been further determined that as the distancearound the liquor collection headers under the screens is increased itbecomes increasingly more difficult to compensate for pressure dropsdown the header as more liquor is picked up from the bottom of eachscreen, which problem is accentuated as the header scales up. It hasbeen further determined that in the period between acid cleanings, theheaders gradually plug up with accumulated scale and debris, and thisaccumulation is nonuniform and is also worse at the point of lowestvelocity. Finally, it has been recognized that because of scaling,screens are normally installed that have much more screen area than isncessary to support the design flow; when the screens are clean, theliquor can choose its own path because of larger area available, and asthe screens scale the scaling and deposits prevent an even flowdistribution, thus there is always some problem with distributionwhether the screens are clean or scaled up. This flow distributionproblem is further aggravated since the screen area varies directly asthe diameter, whereas the tonnage (and required circulation flow) variesas the square of the digester diameter, and for larger digesters theonly way to hold the loading per unit screen area down would be to makethe screens very tall, which involves considerable expense for both thescreens and the digester shell.

According to the present invention, the flow distribution problems thatresult in decreased product unformity and quality have been solved in asimple and efficient manner. According to the present invention, insteadof providing one liquor outlet with each header a pair ofcircumferentially spaced (e.g., 180°) outlets are provided with eachheader, no restrictions being provided in the headers preventing flowfrom one portion of the header to another; and a method of withdrawingliquor from the outlets is provided that ensures that a header of eachheader pair is rested during each alternate withdrawal step, and thatthe side from which liquor is withdrawn is alternated each successivetime that a header is used. This varies the position from which theliquor is extracted and ensures that no paths are set up inside thedigester where liquor is always pulled only one way, this beingparticularly important as the screen starts to scale over. By switchingfrom one side to the other, the low velocity point in the headers ismoved from cycle-to-cycle and all points of the header are exposed to ahigh velocity at least once per complete cycle. This keeps debris andscaling from occurring in the section of the header which was previouslyonly exposed to low velocity liquor, keeping the header much cleaner andpreventing any channelling effects due to header blockage caused bydeposits, debris and scale from originating. In comparative tests of theswitching apparatus and method according to the invention andconventional arrangements utilizing only one liquor withdrawal outletper header, it was found that increased uniformity and improved productquality unmistakably resulted from the practice of the invention.

In particular, according to the present invention a method ofwithdrawing liquid from a pair of vertically spaced annular screens in avertical vessel is provided. Each screen has an annular headerassociated therewith and two circumferentially spaced liquid outlets areassociated with each header, so that first and second headers eachhaving first and second outlets are provided. The method comprises thesteps of substantially sequentially withdrawing liquid from the firstheader first outlet; terminating withdrawal of liquid from the firstheader first outlet and withdrawing liquor from the second header firstoutlet; terminating withdrawal of liquid from the second header firstoutlet and withdrawing liquid from the first header second outlet;terminating withdrawal of liquor from the first header second outlet andwithdrawing liquor from the second header second outlet; and repeatingthe above steps with appropriate frequency to minimize header blockagecaused by deposit of debris and scale. Preferably, the first outlets ofthe first and second headers are somewhat circumferentially spaced fromeach other, and the second outlets of the first and second headers aresomewhat circumferentially spaced from each other. Also, preferably asecond pair of annular screens with associated headers and liquoroutlets are provided vertically spaced from the first pair, with awithdrawal sequence from the second pair liquor outlets substantiallycoincident with the withdrawal from the first pair corresponding liquoroutlets.

Further according to the present invention, a continuous digester forcellulosic pulp material is provided including a vertically extendingelongated vessel; a pair of circumferentially spaced annular screensdisposed in the vessel, each having an annular header associatedtherewith; a first liquid outlet associated with each header; means forwithdrawing liquid from the vessel operatively connected to each outlet;means for heating the withdrawn liquid and returning it to the interiorof the vessel; and means for selectively providing withdrawal from agiven outlet while preventing withdrawal from the other outlets. Theimprovement comprises a second liquid outlet provided with each headerand circumferentially located approximately 180° from the first outletprovided with each header, the second outlets operatively connected tothe liquid withdrawing and selective withdrawal providing means so thatliquid is withdrawn from only one said outlet at a time while withdrawalfrom the other said outlets is prevented. Such apparatus is particularlyeffective in the digester having a diameter of about 14' or greater.

It is the primary object of the present invention to provide a methodand apparatus for the withdrawal and recirculation of cooking liquor incontinuous digesters to minimize header blockage caused by depositeddebris and scale, and to increase pulp uniformity and quality. This andother objects of the invention will become apparent from an inspectionof the detailed description of the invention, and from the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of the relevant portions of a continuousdigester utilizing an exemplary withdrawal system according to theprevent invention;

FIG. 2 is a top sectional view taken along the digester of FIG. 1 at oneof the headers thereof; and

FIG. 3a through 3d are schematic perspective views, with portions cutaway for clarity, illustrating a sequence of exemplary steps forpracticing a method according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

An exemplary continuous digester with which the switching systemaccording to the present invention is employed is illustrated generallyat 10 in the drawings. The digester includes a pair of circumferentiallyspaced annular screens 12, 13, disposed therein, each having an annnularheader 14, 15 associated therewith. As illustrated most clearly in FIG.2 and FIGS. 3a through 3d, preferably each header provides asubstantially circumferentially continuous cavity, being free ofcircumferential dividing members. Preferably, a second pair ofvertically spaced annular screens 16, 17 are also provided in the vessel10, vertically spaced from the first pair of screens 12, 13, and havingheaders 18, 19 associated therewith.

According to the present invention, each annular header 14, 15, 18, 19has first and second liquid outlets associated therewith, the liquidoutlets being circumferentially spaced around the digester 10(preferably spaced 180° apart). First outlets 21, 23, 25 and 27 areassociated with the headers 14, 15, 18 and 19, respectively, as aresecond outlets 22, 24, 26 and 28, respectively.

Means are provided for withdrawing liquid from the vessel 10, such meansbeing operatively connected to each outlet 21 through 28. Suchwithdrawing means preferably takes the form of one or more pumps 32, 33.The pumps 32 and 33 are connected up to means for heating the withdrawnliquid and returning it to the interior of the vessel 10, such meanstaking the form of conventional heaters 35, 35' and central distributionpipes 36, 36' for the upper and lower screen sets respectively. There isfurther provided means for selectively providing withdrawal of liquidfrom a given outlet while preventing withdrawal from the other outlets.Such selective withdrawal providing means include a plurality ofvalves--such as solenoid controlled valves--40 through 47, oneassociated with each outlet 21 through 28, with a common electriccontrol element 49 for selectively controlling operation of the valves40 through 47 according to a predetermined sequence. During operation ofthe digester 10, cellulosic chips material preferably maintain a level L(see FIG. 1), chips being in contact with the screens 12, 13, 16 and 17during operation of the vessel 10.

The practice of the method according to the present invention is seenmost clearly with reference to FIGS. 3a through 3d. For clarity ofillustration in FIGS. 3a through 3d, the first and second outlet pipes(e.g., 21, 22) associated with each header (e.g., 14) are shwon atsubstantially less than 180° apart, although normally the outlets willbe spaced approximately 180° apart. In practicing the method accordingto the present invention, each header is alternately rested insuccessive withdrawal steps, and the side from which the liquor iswithdrawn each successive time that a particular header is used isalteranted.

With reference to FIGS. 3a through 3d, an exemplary method according tothe present invention comprises the steps of substantially sequentially(a) withdrawing liquid from the first header 14 first outlet 21; (b)terminating withdrawal from the outlet 21 and withdrawing liquid fromthe second header 15 first outlet 23; (c) terminating withdrawal ofliquid from the outlet 23 and withdrawing liquid from the first header14 second outlet 22; (d) terminating withdrawal of liquid from theoutlet 22 and withdrawing liquid from the second header 15 second outlet24; and (e) repeating steps (a) through (d), and with appropriatefrequency, to minimize header blockage caused by deposited debris andscale. The control mechanism 49 controls the valves 40, 41, 42, and 43to effect the above withdrawal sequencing, each withdrawal step beingpracticed for a few minutes before sequencing, under normalcircumstances. The method according to the present invention isapplicable whether the first outlets 21, 23 are on the same side of thedigester 10, are located on opposite sides thereof, or are otherwisecircumferentially spaced (and likewise for the second outlets 22, 24),or are circumferentially coincident.

The method according to the present invention is preferably practicedwith the third and fourth headers 18, 19 also being provided, the thirdand fourth headers 18, 19 having first outlets 25, 27 associatedtherewith, and second outlets 26, 28 associated therewith. Utilizingsuch third and fourth headers, the method comprises the further steps of(f) substantially coincident with step (a), withdrawing liquid from thethird header 18 first outlet 25; (g) substantially coincident with step(b), withdrawing liquid from the fourth header 19 first outlet 27 whileterminating withdrawal from the outlet 25; (h) substantially coincidentwith step (c), withdrawing liquid from the third header 18 second outlet26 while terminating whitdrawal from outlet 27; (i) substantiallycoincident with step (d), withdrawing liquid from the fourth header 19second outlet 28 while terminating withdrawal from the outlet 26; and(j) repeating steps (f) through (i) substantially coincidentally withsteps (a) through (d), respectively, and with appropriate frequency tominimize header blockage by deposited debris and scale. The commoncontrol mechanism 49 controls coincident operation of the valves 40, 44;42, 46; 41, 45; and 43, 47, to effect the above sequencing of withdrawalsteps.

Thus, it will be seen that according to the present invention a simpleswitching system and method have been provided which minimize headerclogging due to debris and scale deposition, and which improve pulpquality and uniformity compared to conventional arrangements. While theinvention has been herein shown and described in what is presentlyconceived to be the most practical and preferred embodiment thereof, itwill be apparent to those of ordinary skill in the art that manymodifications may be made thereof within the scope of the invention,which scope is to be accorded the broadest interpretation of theappended claims so as to encompass all equivalent methods andstructures.

What is claimed is:
 1. A method of withdrawing liquid from a pair ofvertically spaced annular screens in a vertical vessel, each having anannular header associated therewith, and two circumferentially spacedliquid outlets being associated with each header, so that first andsecond headers each having first and second outlets, are provided, eachheader providing substantially circumferentially continuous cavity freeof circumferential dividing members; said method comprising the steps ofsubstantially sequentially(a) withdrawing liquid from the first headeroutlet; (b) terminating withdrawal of liquid from the first header firstoutlet and withdrawing liquid from the second header first outlet; (c)terminating withdrawal of liquid from the second header first outlet andwithdrawal liquid from the first header second outlet; (d) terminatingwithdrawal of liquid from the first header second outlet and withdrawalliquid from the second header second outlet; and (e) repeating steps(a)-(d), and with appropriate frequency to minimize header blockagecaused by deposited debris and scale.
 2. A method as recited in claim 1wherein a second pair of vertically spaced annular screens are providedin the vertical vessel, vertically spaced annular screens are providedin the vertical vessel, vertically spaced from the first pair having anannular header associated therewith and two circumferentially spacedliquid outlets being associated with each header, so that third andfourth headers each having first and second outlets, are provided, eachheader providing a substantially circumferentially continuous cavityfree of circumferential dividing members; said method comprising thefurther steps of(f) substantially coincident with step (a), withdrawingliquid from the third header first outlet; (g) substantially coincidentwith step (b), withdrawing liquid from the fourth header first outletwhile terminating withdrawal from the third header first outlet; (h)substantially coincident with step (c), withdrawing liquid from thethird header second outlet while terminating withdrawing from the fourthheader first outlet; (i) substantially coincident with step (d),withdrawing liquid from the fourth header second outlet whileterminating withdrawal from the third second outlet; and (j) repeatingsteps (f)-(i) substantially coincidently with steps (a)-(d),respectively, and with appropriate frequency to minimize header blockageby deposited debris and scale.
 3. A method as recited in claim 2 whereinsaid first through fourth headers are disposed in vertically descendingorder.
 4. A method as recited in claim 1 wherein the vessel is adigester with cellulosic chips in contact with the screens, andcomprising the further steps of heating the withdrawn liquid andreturning it to the interior of the vessel for treatment of cellulosicchips therein.